1. Field of the Invention
This disclosure generally relates to electrical power systems, and more particularly to electrical power systems comprising one or more hybrid power modules, the hybrid power modules comprising, for example, a fuel cell stack and energy storage device.
2. Description of the Related Art
Electrochemical fuel cells convert fuel and oxidant to electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are electrically coupled in series to form a fuel cell stack having a desired power output.
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have flow passages to direct fuel and oxidant to the electrodes, namely the anode and the cathode, respectively. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant, and provide channels for the removal of reaction products, such as water formed during fuel cell operation. The fuel cell system may use the reaction products in maintaining the reaction. For example, reaction water may be used for hydrating the ion exchange membrane and/or maintaining the temperature of the fuel cell stack.
In most practical applications, it is desirable to maintain an approximately constant voltage output from the fuel cell stack. One approach is to employ an energy storage device such as a battery or ultra-capacitor electrically coupled in parallel with the fuel cell system as a hybrid power module, to provide additional current when the demand of the load exceeds the output of the fuel cell stack and to store current when the output of the fuel cell stack exceeds the demand of the load, as taught in commonly assigned pending U.S. patent application Ser. No. 10/017,470, entitled “Method and Apparatus for Controlling Voltage From a Fuel Cell System”; Ser. No. 10/017,462, entitled “Method and Apparatus for Multiple Mode Control of Voltage From a Fuel Cell System”; and Ser. No. 10/017,461, entitled “Fuel Cell System Multiple Stage Voltage Control Method and Apparatus”, all filed Dec. 14, 2001. Thus, the energy storage device provides the ability to accommodate starting, bridging and surging power requirements. While the energy storage device could be charged while the fuel cell stack produces power, charging from an external source when the fuel cell stack is not operating has required an external equalizer.
As taught in commonly assigned pending patent applications, it is also desirable to provide redundancy for arrays of hybrid power modules, electrically coupled in series and/or parallel. Providing redundancy is complicated by the possibility of a shorted cell of an energy storage device dragging down other energy storage devices electrically coupled in parallel with the malfunctioning energy storage device. It is also desirable to allow the use of different types of energy storage devices in an array of hybrid power modules, for example, different energy storage devices (e.g., batteries and ultra-capacitors), different battery chemistries (e.g., lead acid, nickel metal hydride, nickel cadmium, lithium ion), energy storage devices of different ages, and/or energy storage devices produced by different manufacturers. It is further desirable to allow exchanges (“hot swappable”) of working hybrid power modules and/or energy storage devices for malfunctioning hybrid power modules and/or energy storage devices without having to power down the array.
The many different practical applications for fuel cell based power supplies require a large variety of different power delivery capabilities. In most instances it is prohibitively costly and operationally inefficient to employ a power supply capable of providing more power than required by the application. It is also costly and inefficient to design, manufacture and maintain inventories of different power supplies capable of meeting the demand of each potential application (e.g., 1 kW, 2 kW, 5 kW, 10 kW, etc.). Further, it is desirable to increase the reliability of the power supply, without significantly increasing the cost. It is also costly and inefficient to design, manufacture and maintain different external equalizers to accommodate the various customer requirements.
Thus, a less costly, less complex and/or more efficient approach to fuel cell based power supplies, such as hybrid power modules is desirable.